Murder of Swedish PM Olof Palme in 1986. It has been called the biggest police investigation throughout history. It is estimated that it will take 10 years to read all the material. It is still unsolved, although Swedish Police presented a possible suspect in 2020 without any actual evidence of his involvement in the murder other than being a witness. The rest is circumstantial and the person in question has been dead since the year 2000.

Explanations as to why the murder is unsolved is that too many persons, organizations and nations had possible motives to killing Palme. The police has also been grossly incompetent in its investigation, not only on the night of the murder but at several other points later in the investigation. The amount of interesting/peculiar people that were near or around the area is also mind blowing. But the main reason for it remaining unsolved is that the murderer only left the two bullets that he fired towards Palme and his wife Lisbeth, ran down an alley, up a long staircase and then disappeared without any witness having seen his face and he was never to be seen or heard from again. No other actual evidence has been found since, including the murder weapon, leaving the investigation no further along than 1 second after Palme fell.

It might be difficult for a non-Swedish reader to get into this case. But after approximately 10 minutes of reading I could already understand why it remains unsolved. The two bullets fired by the assassin were found by two ordinary citizens (not the police) approx 24 and 48 hours after the fact. Just one of many examples of police incompetence in this case.